Evaluation of sequence variation and selection in the bindin locus of the red sea urchin, Strongylocentrotus franciscanus

Recent evidence suggests that gamete recognition proteins may be subjected to directed evolutionary pressure that enhances sequence variability. We ev aluated whether diversity enhancing selection is operating on a marine inve rtebrate fertilization protein by examining the intraspecific DNA sequence variation of a 273-base pail region located at the 5' end of the sperm bind in locus in 134 adult red sea urchins (Strongylocentrotus franciscanus). Bi ndin is a sperm recognition protein that mediates: species-specific gamete interactions in sea urchins. The region of the bindin locus examined was fo und to be polymorphic with 14 alleles. Mean pairwise comparison of the 14 a lleles indicates moderate sequence diversity (p-distance = 1.06). No eviden ce of diversity enhancing selection was found. It was not possible to rejec t the null hypothesis that the sequence variation observed in S. franciscan us bindin is a result of neutral evolution, Statistical evaluation of expec ted proportions of replacement and silent nucleotide substitutions, observe d versus expected proportions of radical replacement substitutions, and con formance to the McDonald and Kreitman test of neutral evolution all indicat e that random mutation followed by genetic drift created the polymorphisms observed in bindin. Observed frequencies were also highly similar to result s expected for a neutrally evolving locus, suggesting that the polymorphism observed in the 5' region of S. franciscanus bindin is a result of neutral evolution.